automatic memory management

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reference counting

reference counting

• every variable has a reference-count(RC) field
• RC is the number of references to the variable
• when RC reaches 0 the variable is deallocated

Object a = new Object(); // let's call it A
// RC(A) = 1
Object b = new Object(); // let's call it B
// RC(B) = 1
b = a;
// RC(A) = 2
// RC(B) = 0 -> B is deallocated

pros

• predictable performance
• smooth execution without interruptions
• implementable in:
  • manual memory management system
  • automatic memory management system
• cost is proportional to actual computation

cons

• cannot deal with circular structures
• is generally slow incurring a huge “write barrier”

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garbage collection

garbage collection

deallocation is the responsibility of the language runtime system, not the programmer’s

• programmer never deallocates memory
• a GC procedure collects unused variables from time to time
• GC is found in: Java, Python, ML, Lisp, …

mark & sweep

• mark unmark all cells
• sweep mark all cells in use (stack, global variables), and cells which can be accessed (directly or indirectly) from them
• release release all unmarked cells